1. Field of the Invention
The present invention relates to a received data recovering device, for example, a device for receiving a baseband signal of serial data, or the like.
2. Description of the Related Art
The radio communication standard called Bluetooth (™) uses a 2.4 GHz radio frequency band. This band is a frequency band with no license required, and has been used in the IEEE (Institute of Electrical and Electronics Engineers) 802.11b standard employed in radio LANs (Local Area Networks) and the like, and also used for radiowaves produced in microwave ovens. Inasmuch as it is used in devices or apparatuses over a wide range as noted above, a possibility can not be avoided that radiowaves emitted from the devices or apparatuses interfere with each other. Therefore, when using this band, an influence of radiowave interference should be taken in account.
As is common in many radio systems, a signal of the Bluetooth standard is down-converted from an RF (Radio Frequency) band to an intermediate frequency band upon reception at a receiving device via a radio territory (space). Then, the receiving device demodulates the down-converted signal, and quantizes or digitizes the demodulated signal in an amplitude direction. Through the foregoing successive processing, the receiving device recovers only the binary digital data.
In this event, a clock signal for sampling the data in the receiving device is not transmitted from a transmitting device along with the data to be recovered. Therefore, the receiving device extracts a clock component from the received data in a baseband receiving section. By sampling the received data using the recovered clock signal, the receiving device converts the temporally continuous data into data of a format that can be handled as information.
There are available many methods for extracting the clock component from the received data. In general, the clock component is extracted by sampling several times change points of the received data and averaging obtained phase information of the change points. In the receiving device, the thus simply extracted clock signal has been used as sampling clocks to sample the received data.
The foregoing clock extracting method, is suitable to sample, for example, such received data whose change point phase does not vary widely in terminals or the like. There are those instances, however, where due to an influence of radiowave interference or the like, the clock extraction of the Bluetooth standard is carried out when large changes occur in phases exhibited by the change points of the received data, or at independent timings in individual devices. The clocks produced through such an operation cause limitation to the performance of the terminals of the Bluetooth standard. Further, if a system that implements communications according to the Bluetooth standard is constructed using those terminals, the system can not fully demonstrate the performance relative to a specification of the system, and thus the performance limit is caused.
Further, since the terminal aims to perform the clock extraction from a small change in phase, there are those instances where even if the terminal detects a synchronization word located at a beginning part of the received data, when a rapid phase change occurs thereafter, it can not follow this phase change. In this event, in the terminal, if more than a correctable number of errors are detected through error detection processing applied to an intermediate or final part of the received data, this received data can not be received as effective data. As a result, the terminal needs to request retransmission of the data to a communication counterpart device or apparatus, and thus the data transmission efficiency is lowered.